1. Field of the Invention
This invention relates to a process for improving high-temperature alloys, and more specifically, to the electron beam evaporation and deposition process which produces high purity and ultrafine grain size in high-temperature and reactive metal alloys.
2. Prior Art
Alloys made of high melting-point nickel, cobalt and iron base metals are used in aircraft gas turbines for blades, vanes and disks which operate under high-cycle fatigue conditions at metal temperatures up to 1250.degree. C. For these gas turbine components to have long service life and low maintenance, the most important material property is high purity, i.e., the smallest possible size and number of nonmetallic inclusions, where cracks can start with resulting shortened service life under the prevailing high-cycle fatigue operating conditions. The next most important property for such alloys is ultrafine grain size which gives the alloys superplasticity and increased formability, such as ease in forging. It is desirable for high-temperature alloys to possess both high purity and ultrafine grain size.
Various processes of the melting and remelting (or refining) type have been devised for producing high-temperature alloys of high purity, and the principal ones are briefly characterized below:
Vacuum Arc Remelting (VAR) uses a molten pool with heat extraction from the bottom of the pool, resulting in directional solidification. Vacuum Arc Double Electrode Remelting (VADAR) employs two consumable electrodes with their ends facing each other, which approach each other during melting so that the molten metal droplets fall into a mold beneath the electrodes. Electroslag Remelting (ESR) is a process where a consumable electrode is remelted under a slag that provides resistance heating.
Vacuum Induction Melting (VIM) permits particles to spend a greater amount of time in the molten metal pool, but is open to reactions between the melt and the refractory crucible. Induction Slag Melting (ISM) uses a "crucible" consisting of a thin frozen shell formed on the inside walls of a copper tube, with side feeding to the melts. Electron Beam Melting (EBM) allows a variety of feed materials to be used (including sponge, scrap and chips) and does vaporize unwanted gases and volatile trace elements.
Plasma Melting (PM) has many advantages, such as a high degree of control of the plasma (temperature, atmosphere, flow rate), and can use lower grade starting materials. Electron Beam Cold Hearth Refining (EBCHR) uses an electron beam to melt but not vaporize an electrode or feed which falls into a cold mold with the aim of achieving smaller grain sizes in the ingot produced. Laser Beam Melting (LBM) has been used in laboratories to produce high temperature alloys.
Current practice is to start with a VIM ingot, and then use one or two of the above-named processes in series. The second steps are VAR, VADAR, PC, ESR or EBCHR. A three-step process uses ESR followed by VAR.
Of these processes, the ones producing smaller inclusions which significantly increase the fatigue life are: VIM/VAR, VIM/ESR and VIM/EBCHR. The quantitative purity, in terms of largest inclusion diameter, and the associated fatigue life obtained with the last-named three two-step processes are shown for a nickel base alloy containing chromium, cobalt, molybdenum and other additions, known as Rene 95 in the following table:
______________________________________ Largest inclusion Cycles to fatigue Process diameter, micron meters failure ______________________________________ VIM/VAR 25 20 VIM/ESR 11 100 VIM/EBCHR 8 200 ______________________________________
While these process developments have resulted in a reduction of inclusion size and extention of fatigue life, further improvements are needed. A case in point is high-temperature alloys of titanium used in rotating parts of jet engines. These alloys have been routinely triple melted, and yet the occurrence of so-called hard alpha Type 1 areas, which are high in concentration of the undesirable elements carbon, nitrogen or oxygen which lead to failure at lower stresses than in alloys without such areas, could not be avoided and are believed to have caused at least six jet engine failures. Currently, some of the melting processes mentioned above, such as VAR, EBM, and EBCHR, are being tried to reduce Type 1 areas, but unsuccessfully to date.
The melting and remelting processes have attained some success in achieving high purity and longer fatigue life, but have not produced adequate ultrafine grain sizes. Toward this end powder compaction (PC) has been used, wherein the alloy is made in powder form and the consolidated into an ingot or disk. Control of the powder grain size permits an ultrafine grain structure to be obtained.
The present state of art of high temperature alloy processing has produced ultrafine grain structure by using the powder metallurgy approach. By producing powders of these alloys by processes that rapidly solidify the metal into very small particles, ultrafine grain sizes have been realized. The alloys have superplastic properties and are formed into final products with much less difficulty than conventional larger grained materials. However, none of the powder metallurgy processes attain the high purity of the melting processes and the associated increased fatigue life.
Thus, it is seen that there is a need for a process which will provide a high-temperature alloy exhibiting high purity and ultrafine grain structure.